So, "mail" in /var and "the webs" in /var/www. I know its a lot of space for mail which probably never will be used. But what the heck! It could of course be an idea to make /var/www on the remainder of the 120 Gb, and /var on the remainder of the 80 Gb disk. I mean it will probably be more "web" then "mail" if you get my drift.

I tend to recommend a single large partition for the OS during the period where new platforms are being initially provisioned and staged, and through initial testing and burn-in. This way, a more accurate assessment of OS storage partitioning can be derived. Every OS implementation is different, and OS storage requirements can differ dramatically, depending on needs. If you are building ports, for example, you could easily consume 40GB just for port building, even with bulk package building enabled, which erases working areas as they are no longer needed.

For small servers and workstations with single drives (or single RAID arrays) the entire environment can be placed in a single partition until storage needs can be accurately assessed, then a reconfiguration can be executed, typically via backup/restore.

For servers with large application-managed storage requirements, my recommendation is to keep those partitions separate from the OS. For example, for a large mail server using Sendmail, I recommend keeping /var/spool mounted separately from the OS. For a large webserver, I would recommend keeping /var/www mounted separately in the same manner. However, for webservers that serve data from a back-end DBMS, the separation might be for /var/postgresql rather than /var/www. My point here is the separation of application-managed data from the OS will be application dependent.